Conventional laser mice collect light reflected at a specular reflection angle. Highly specular surfaces do not scatter very much light and can produce a bright (i.e., high intensity) image. When used on highly polished or glossy surfaces (e.g., glass), the signal intensity of the reflected laser light is greater than that of a similar signal from LED mice. However, the contrast of the image is low because of the high intensity of the reflected light signal. The relatively low contrast makes it difficult to distinguish individual features on the surface and, hence, makes it difficult to determine movements of the device based on relative movements of the features in sequential navigation images.
FIG. 1 illustrates an arrangement for a conventional laser optical navigation device 10. The conventional laser optical navigation device 10 includes a light source 12 and an image sensor 14. The light source 12 directs laser light toward a specular navigation surface 16. The incident laser light approaches the specular navigation surface 16 at an angle of incidence, θ. The laser light reflects off of the specular navigation surface 16 at an angle of reflection, θ, which is equal to the angle of incidence, θ, because of the specular nature of the specular navigation surface 16. The image sensor 14 receives the reflected light and, over time, generates a sequence of images of the specular navigation surface 16. The conventional laser optical navigation device 10 also includes an imaging lens 18 to more or less focus the reflected laser light on the image sensor 14.
Due to the specular nature of the specular navigation surface 16, substantially all of the light that is incident on the specular navigation surface 16 is reflected toward the image sensor 14. Also, the reflected light does not exhibit a significant pattern because of the uniformity (i.e., low contrast) of the specular navigation surface 16. Therefore, the high intensity of the reflected light and the low contrast of the resulting image based on the reflected light render relatively little detail for use in determining movements of the conventional laser optical navigation device 10. This lack of detail in the resulting images makes it difficult to accurately determine the movements of the conventional laser optical navigation device 10 relative to the specular navigation surface 16.